The present invention is directed to a small-sized print head and, in particular, to a small-sized ink jet print head for use in an ink-on-demand printer.
Ink jet print heads for use in ink-on-demand type printers are gaining increasing acceptance. Such ink jet print heads include a pressurization chamber, the volume of which is reduced by mechanical distortion caused by a piezoelectric element, in order to eject droplets of liquid ink through a nozzle communicating with the pressurization chamber. The energy required for printing in ink jet print heads is small and such print heads can be provided with multiple nozzles. Although the structure of ink jet print heads which eject ink droplets is relatively uncomplicated, such print heads have not been completely theoretically analyzed since ink ejection is produced under transient conditions and the pressure, rate of flow and the like are difficult to measure due to the small size of the print head.
In highly compact multiple-nozzle print heads wherein, for example, twenty-four (24) or more nozzles are required for printing Chinese characters, each individual pressurization chamber and associated piezoelectric element should preferrably be small in size. However, heretofore, it has not been clear as to how the pressurization chamber can be reduced in size due to incomplete theoretical analysis. Piezoelectric elements have been utilized having a thickness tp.apprxeq.0.3 mm or more and a diameter d.apprxeq.5 mm or greater. Furthermore, piezoelectric elements which are small in size generate a small driving power and require an increased drive voltage thereby making such elements practically infeasible.
For example, Stemme et al in IEEE, Transaction on Electron Devices, ED-20 No. 1,14(1973) suggested the arrangement where tp=0.3 mm and d=5 mm. In preprint No. 6, preprint collection for the 8th National Conference, 1980, of the Picture Image Electronics Society, Matsuda et al. disclosed that a rectangular piezoelectric element having tp=0.3 mm has the best ratio of mechanical distortion. The size of such a piezoelectric element is assumed to be about 2 mm.times.15 mm and, hence, such a large sized piezoelectric element is unsatisfactory where it is desired to reduce the size of the piezoelectric elements.
The larger the area of the piezoelectric element, the higher the cost becomes for a substrate constituting the piezoelectric element and the print head. A highly compact ink jet print head is therefore disadvantageous since it comprises a number of piezoelectric elements. With multiple nozzles, an increase in the size of a piezoelectric element results in a greater distance between the distal end of the nozzles and the pressurization chamber and hence, in an increased resistance in the flow passage. Such an increased flow resistance, in turn, necessitates an increase in the area of the piezoelectric element to gain a greater driving power, a disadvantage caused by the size and driving power of the piezoelectric elements. Accordingly, an ink jet print head which is small in size without requiring an increased drive voltage to operate, is extremely desired.